Scholars describe the one-drop rule—the idea that any African ancestry makes a person black—as the American regime of race. While accounts of when the rule emerged vary widely, ranging from the 1660s to the 1920s, most legal scholars have assumed that once established, the rule created a bright line that people were bound to follow. This Article reconstructs the one-drop rule’s meaning and purpose from 1600 to 1860, setting it within the context of “racial migration,” the continual process by which people of African descent assimilated into white communities.

While ideologies of blood-borne racial difference predate Jamestown, the rhetoric of purity was always undermined by the realities of mixture. Cases such as Hudgins v. Wright and State v. Cantey simultaneously strengthened and undermined the color line, expressing confidence in the permanence of racial difference while allowing people of color to become white. Instead of simply widening the racial divide, restrictions on the liberty and livelihoods of African Americans pushed many into whiteness. As more people became vulnerable to reclassification, courts acted to preserve social stability in Southern communities, shielding them from the insecurity that an insistence on racial purity would engender.

The one-drop rule’s transformation from ideological current to legal bright line and presumed social reality is a story not of slavery, but of freedom. In the 1840s and 1850s, the prospect of emancipation hastened the rule’s spread as whites attempted to preserve property relations in slavery’s absence. In Northern border states such as Ohio, the rule emerged by 1860 as a legal argument and a social attitude. The rule’s ostensible opponents—abolitionists—propagated it, deploying it as a rhetorical weapon to symbolize slavery’s cruelty and to argue that Northern whites could be enslaved. But the rule’s ascent did not make it any more enforceable than it had ever been.